tools/memory-model: Weaken ctrl dependency definition in explanation.txt

The current informal control dependency definition in explanation.txt is
too broad and, as discussed, needs to be updated.

Consider the following example:

> if(READ_ONCE(x))
> return 42;
>
> WRITE_ONCE(y, 42);
>
> return 21;

The read event determines whether the write event will be executed "at all"
- as per the current definition - but the formal LKMM does not recognize
this as a control dependency.

Introduce a new definition which includes the requirement for the second
memory access event to syntactically lie within the arm of a non-loop
conditional.

Link: https://lore.kernel.org/all/[e-mail]/
Cc: Marco Elver <[e-mail]>
Cc: Charalampos Mainas <[e-mail]>
Cc: Pramod Bhatotia <[e-mail]>
Cc: Soham Chakraborty <[e-mail]>
Cc: Martin Fink <[e-mail]>
Co-developed-by: Alan Stern <[e-mail]>
Signed-off-by: Alan Stern <[e-mail]>
Signed-off-by: Paul Heidekrüger <[e-mail]>
Signed-off-by: Paul E. McKenney <[e-mail]>

tools/memory-model/Documentation/explanation.txt

···
 through a pointer will depend on the value read earlier from that
 pointer.
 
-Finally, a read event and another memory access event are linked by a
-control dependency if the value obtained by the read affects whether
-the second event is executed at all.  Simple example:
+Finally, a read event X and a write event Y are linked by a control
+dependency if Y syntactically lies within an arm of an if statement and
+X affects the evaluation of the if condition via a data or address
+dependency (or similarly for a switch statement).  Simple example:
 
 	int x, y;